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A spider, seemingly possessed, spins an uncharacterisitic web—just before wasp larvae nesting on its abdomen suck the last nourishing juices from the spider's dying body and make a cocoon in the weird web.

A worm gets into the brain of a shy, shade-loving snail, compelling it to crawl out of its safe home and into the open where it gets attacked by a bird—which is destined to be the parasite's next host.

Creepy tales like these are happening all over the animal kingdom, and not just on Halloween or under a full moon. Some organisms have taken evolutionary advantage to a whole new level, achieving success by insidiously—and still quite mysteriously—changing the behavior of another animal against its will.View a Slide Show of Zombie Animals

Although not undead in the strictest sci-fi definition of "zombie," these captive creatures nonetheless behave as if possessed by a force from beyond. That force, however, is often controlling them from the inside, making the unfortunate hosts do deadly things.

In the case of the spooked spider (Plesiometa argyra), a parasitic wasp (Hymenoepimecis argyraphaga) lays her eggs on the spider's abdomen. Just before the larva emerges, the host spins a strange, new type of web—one that looks nothing like its usual wide nets. This silk platform, however, is perfectly suited to supporting a cocoon for the vulnerable young wasp larvae, which have been feasting on the spider's innards as they grow.

The snail-manipulating flatworm (Leucochloridium paradoxum) grows and multiplies inside the snail. Once ready to move on to its next host, the worms push up into the snail's tentacles, making them swell and squirm, mimicking the action of bugs that birds like to eat. As the snail crawls, blindly, into the sunlight, a passing bird is likely to swoop down to snatch a tasty tentacle or two. The worm-infested meal will then infect the bird, which passes it onto other snails via dubious droppings.

The mechanisms by which these parasites are commanding their hosts remain, by in large, unsolved mysteries.

"We don't know how it works, but it's obviously some type of chemical," says David Hughes, a researcher at Harvard University. "It's co-opting a preexisting behavior." As part of their normal routine, snails climb stalks and spiders spin webs, but the parasites have managed to take these particular behaviors and mold them to their own advantage—often resulting in their hosts' demise.

"Typically you have behavior that a host would do at some point in its life," adds Edward Levri, an associate professor of biology at The Pennsylvania State University in Altoona. "It's just happening at an odd, nonadvantageous time for the host, to the benefit of the parasite."

Creepy control
So many very different—and very bizarre—examples of parasite-controlled behavior modification have been observed that researchers have a wealth of instances to study. But it is no longer enough to document these parasites. "We have to go beyond describing behavior," Hughes says. "Now people are trying to look at how parasites are changing behavior."

Figuring this puzzle out, however, has proved to be quite difficult. "Modified organisms are more complex than we had previously believed," says Frederic Thomas, a scientist at the Genetics and Evolution of Infectious Diseases research group in Montpellier, France, and the department of biology at the University of Montreal.

With improved technology, researchers in this freaky field have been able to start sequencing the genomes and parsing proteins of some of these parasites and hosts to unravel where behavioral changes happen. "In at least some cases, some of these parasites produce neurotransmitters or hormones that mimic host hormones," Levri says.

Not all of the parasites, however, are traditional body snatchers.

"You can easily understand that a parasite can control the behavior when it is inside," Thomas says. But how do some manage to dictate a host's behavior when they're not, technically, hosting a parasite? "To me, it's like magic," he adds.

The aberrant web-spinning P. argyra demonstrates this to some extent, the parasitic wasp eggs having been laid on the outside of the spider's body. In another bizarre example of external control, a different species of wasp (Ampulex compressa) is able to control a cockroach—via an injection into its brain—and force it to enter its nest to become food and shelter for the wasp's larvae.
Are humans next?
Even people may not be fully immune to such manipulation. Much of our modern-day behavior seems to suit the interests other species, such as hitchhiking invasive species and airline-riding viruses that hop continents in a matter of hours. But can we also be manipulated from within?

To a certain degree, yes. Perhaps the most familiar example is rabies, which can cause human hosts—like other mammals—to salivate excessively, thereby spreading the deadly virus. Researchers have even shown that malaria-infected people are more attractive to mosquitoes, which spread the infected blood to another host, according to a 2005 PLoS ONE paper.

Such pernicious parasitism is indeed widespread throughout various kingdoms. "We know it's evolved a few times," Exeter's Hughes says. So, he wonders: "Has it been different solutions to the same problem?"

In the quest to understand both how these uncanny behaviors work and how they emerged, researchers are also enlisting the help of evolutionary ecology. They are examining these behaviors on a smaller scale. For example if two larvae are growing on a controlled host, like the P. argyra orb-web spider, what happens to the spider's behavior when one is mature and the other isn't yet ready to emerge?

Some hosts have evolved to cope with frequent parasitism. For example, one beetle that is frequently killed off early by a parasite has adapted by increasing its sexual activity and producing more offspring, Thomas notes.

But the chilling question remains: How can bugs control other bugs? And in some of the more extreme examples, Hughes ponders: "How can a member of one kingdom modify the behavior of another kingdom?" Until scientists find that answer, it's all eerie action, for sure.